1. Field of the Invention
The present invention relates generally to thermal printers, and, more particularly, to a method for controlling a thermal printer so as to increase the effective output resolution of the thermal printer, and thereby increase the visual quality of the output image printed by the thermal printer.
2. Background of the Invention
It is well known that the resolution of a printer will largely determine the quality of the output image produced by that printer. The higher the resolution in terms of dots per inch (dpi), the better the quality. For printers that have native resolutions greater than about 1000 dpi, it is difficult for the human eye to discern individual dots or pixels in the printed output. For printers with native resolutions below about 1000 dpi, the appearance of discernable dots or pixels can reduce the overall quality of the output image. As would be expected, however, the cost and complexity of higher resolution printers having native resolutions greater than about 1000 dpi is significantly greater than for lower resolution printers having native resolutions less than about 1000 dpi. Consequently, numerous techniques have been developed in an attempt to increase the image quality of lower resolution printers so as to more closely simulate the image quality of more expensive higher resolution printers.
Unlike other types of printers, such as laser printers or ink jet printers, there are presently no thermal printers which are capable of producing high resolution outputs that are greater than 1000 dpi. Currently, the highest resolution thermal printers produce images of about 400 dpi by advancing a medium past a row or line of resistive heating elements arranged, for example, in a print line positioned across a page to be printed. Individual resistive heating elements in the print head are activated by an input pulse to produce a single print line of pixel images on a print medium. The resulting image is recorded on the print medium either by thermal activation of a marking, material embedded in the medium or by thermal transfer of a wax or film onto the medium. The vertical output resolution of such a thermal printer is a function of how far the print medium is advanced past the print head for each print cycle, but the horizontal output resolution of the thermal line printer is dictated by the physical pitch or center-to-center distance between adjacent heating elements in the print head. Because the pitch is a physical parameter of the print head, the horizontal output resolution effectively becomes the fixed resolution for a thermal printer.
Due to a variety of limitations involved in creating very small, yet very precise, resistive heating elements, it has been impractical to construct a thermal print head with resistive elements at pitches greater than about 400 dpi. For example, one of the biggest problems in creating higher density print heads has been connecting each heating element with its corresponding driver circuit such that the heating elements, driver circuits and connections can all be positioned on the same substrate. Though these problems may be solved in the future to allow for production of higher resolution, more expensive print heads, the native resolution of existing thermal printers is limited to values below about 400 dpi. While this would seem to provide even more reason to attempt to increase the image quality of thermal printers, the focus of most efforts to enhance thermal printers has been to improve the output consistency, and not the effective output resolution, of thermal printers.
A great many techniques have been developed to control thermal print heads so as to produce consistent and reliable output images with high pixel-to-pixel consistency between successive print lines. Various hysterisis control techniques have been developed to correct for differences in the temperature of the print head between successive print cycles, as shown, for example, in U.S. Pat. Nos. 4,859,093 and 5,235,345. Similarly, various techniques have been developed to correct for end of line differences encountered in the use of carriage-type thermal print heads, as shown, for example, in U.S. Pat. Nos. 4,789,870 and 4,872,772. Another technique to compensate for residual temperature differences in the print head uses a staggered print head with alternating even and odd print pixels, as shown, for example, in U.S. Pat. No. 4,779,102. While all of these techniques may improve the pixel-to-pixel consistency between successive print lines printed by a thermal printer, none of these control systems can offer an increase in the effective output resolution of the thermal printer.
In an effort to increase the apparent resolution of thermal printers, grey scale or gradiated thermal printers have been used, as shown, for example, in U.S. Pat. Nos. 4,524,203 and 5,216,440. In a grey scale printer, the native pixel resolution of the thermal printer is not increased; instead, a visual illusion of higher resolution is produced by printing each of the pixels at different optical density levels, and then relying on the human eye to perform the smoothing operation as the image is viewed. As a result, the effective output resolution of the image is not actually increased, only its apparent resolution is increased. In other words, there is no actual change in the size of the pixel images that are printed, only the optical density of the pixel images is altered in order to obtain a perceived increase in image quality. The problem with this approach is that special grey-scale print media are required in order to reproduce grey-scale images, as opposed to binary images. Consequently, in those situations where a true binary output image is required, such as when a master print for a printing press operation is desired, a grey scale output image is not acceptable.
Although no techniques have been developed to increase the effective output resolution of thermal printers, techniques have been developed for other types of low resolution printers to increase the effective output resolution of those devices. For lower resolution laser printers, for example, three of the most popular resolution enhancement techniques are represented by U.S. Pat. Nos. 5,041,848 issued to Gilbert et al., U.S. Pat. No. 5,193,008 issued to Frazier et al., and 5,005,139 issued to Tung. In U.S. Pat. No. 5,041,848, the marking engine is modulated by a raster scan line that has a higher horizontal frequency than the native vertical frequency of the marking engine so as to create a non-square pixel image. The output image is smoothed by selectively modifying pixel values on either side of any vertical transition points of an ideal image outline as each scan line is rasterized. In U.S. Pat. No. 5,193,008, the effective resolution of the output image is increased by doubling the resolution of the image in a frame buffer holding the scan lines and then reading a pair of scan lines together and comparing vertically adjacent pixels to generate the raster scan line signal that will actually modulate the marking engine. In U.S. Pat. No. 5,005,139, the smoothing takes place after the scan lines have been rasterized by comparing a sample window from a group of scan lines to a number of matching bit patterns or templates. If a match is found, then the central bit of the sample window is modified in an attempt to smooth the output image by using a narrower output pixel, for example. For a more detailed explanation of these resolution enhancement techniques for laser printers, reference is made to Stiedel, L., "Technology Overview: Resolution Enhancement Technologies for Laser Printers", LaserMaster Corporation, 1991.
While these types of resolution enhancement techniques have proven effective when applied to the marking engines of laser printers, they are not applicable to thermal printers for a number of reasons. Generally, there is very little crossover between techniques that work on laser printers and techniques that work for thermal printers. This is due to the fact that the fundamental marking operations of each printer are so different. In addition, the physical limitations of each printer that affect the output resolution are effectively opposite one another. In a laser printer, the vertical resolution is effectively fixed and the horizontal resolution can be increased by increasing the modulation frequency at which the laser scans the photoconductor drum. In a thermal printer, on the other hand, the horizontal frequency is fixed due to the physical pitch of the heating elements and the vertical resolution is the only resolution which can be altered. As a result, techniques for a laser printer cannot be plugged into a thermal printer with an expectation that similar results will be achieved.
In a laser printer, an electrical charge is produced on a rotating imaging drum by scanning the laser across the drum in response to a modulated output signal representative of the image to be printed. After the charge image is formed on the imaging drum, it is rotated past a toner applicator and attracts oppositely charged toner particle to the drum. Finally, the toner particles on the toner drum are transferred to a print medium, such as a sheet of paper, and the toner is then fixed or fused onto the print medium. The resolution enhancement techniques described above take advantage of the fact that the imaging drum acts as a kind of additive accumulator of charge over time. Consequently, it is possible to create pixel images that are smaller than the native pixel resolution of the marking engine of the laser printer by accumulating charge between successive raster scans, for example, to generate a region of overlapped charge. Due to the relatively long charge retention time of the imaging drum, this region of overlapped charge acts as a large-scale temporal charge accumulator that can be controlled to create pixel images smaller than the native resolution when the toner is attracted to the overlapped charge areas on the imaging drum.
There is nothing in a thermal printer, however, that is equivalent to a large-scale temporal charge accumulator necessary to facilitate the types of resolution enhancement techniques described above. In a thermal printer, the resistive heating elements provide the printing energy and are essentially equivalent to the laser in a laser printer. The marking material or the wax fixes the intended image and is essentially equivalent to the toner and fuser. There is, however, no direct equivalent in a thermal printer to the imaging drum in a laser printer. Consequently, the enhancement techniques used with laser printers are not applicable to thermal printers.
Although thermal printers are now capable of producing consistent and reliable quality output images, particularly at lower resolutions, there has been no effort to increase the effective resolution of such thermal printers. Consequently, it would be advantageous to provide a method and apparatus for controlling thermal printers that could increase the effective output resolution of thermal printers above the native resolution of the print head.